Lovering and Bond on Magnetic Observations at Cambridge. 7 
Abundant materials are furnished by the preceding catalogue of 
transits for deducing the state of the instrument, the direction of the 
meridian and the true sidereal time. By means of these observa- 
tions the rate of the clock is found to be 
8. 8. 
From the 10th to 15th July —1.16 From 15th to 25th July —1.41 
—1.09 —1.43 
—1.15 —1.43 
—1.17 
—1.12 —4,27 
—5.69 Mean —1.42 
Mean —1.14 
—1.42 
—2.56 
—1.28 Mean from 10th to 25th July. 
There are three methods in common use for determining the azi- 
muth of the Transit instrument. One of them employs the succes- 
sive intervals between the upper and lower passage of Polaris; 
another compares the transits of two cireumpolar stars whose right 
ascensions vary about 12 hours; while the last depends upon the 
transits of high and low stars, including north and south stars. 
When the latter method is adopted, it is advisable to select stars 
whose difference of declination is at least equal to 40°. There are 
defects in all these methods. The first supposes the rate of the 
clock to be uniform during the 24 hours; this may not always be 
the case; but on account of the slow motion of the star a consid- 
erable error in time would make but a small difference in the azi- 
muth of the instrument, The second process depends upon the 
